Muscle mechanics

A muscle twitch is a brief, weak contraction produced in a muscle fiber in response to a single action potential. While the action potential lasts 1 to 2 msec, the resulting muscle twitch lasts approximately 100 msec. However, a muscle twitch in a single muscle fiber is too brief and too weak to be useful or to perform any meaningful work. In fact, hundreds or thousands of muscle fibers are organized into whole muscles. In this way, the fibers may work together to produce muscle contractions strong enough and of sufficient duration to be productive. Furthermore, muscles must be able to generate contractions of variable strengths. Different tasks require different degrees of contraction or tension development within the whole muscle. The strength of skeletal muscle contraction depends on two major factors:

• Number of muscle fibers contracting

• Amount of tension developed by each contracting muscle fiber

Number of muscle fibers contracting. As the number of contracting muscle fibers increases, the strength of skeletal muscle contraction increases. Two major factors determine the number of muscle fibers activated at any given moment:

• Multiple motor unit summation

• Asynchronous motor unit summation

A motor unit is defined as an alpha motor neuron and all of the skeletal muscle fibers it innervates. The number of muscle fibers innervated by an alpha motor neuron varies considerably, depending upon the function of the muscle. For example, the muscles of the eyes and hands have very small motor units. In other words, each alpha motor neuron associated with these muscles synapses with only a few muscle fibers. As a result, each of these muscles is innervated by a comparatively large number of alpha motor neurons. Densely innervated muscles are capable of carrying out more precise, complex motor activities. On the other hand, antigravity muscles have very large motor units. For example, the gastrocnemius muscle of the calf has about 2000 muscle fibers in each motor unit. Muscles with large motor units tend to be more powerful and more coarsely controlled.

Multiple motor unit summation involves recruitment of motor units. As the number of motor units stimulated at any given moment increases, the strength of contraction increases. Asynchronous motor unit summation refers to the condition in which motor unit activation within a muscle is alternated. In other words, at one moment, some of the motor units within the muscle are activated, while other motor units are relaxed. This is followed by the relaxation of previously activated motor units and activation of previously relaxed motor units. Consequently, only a fraction of the motor units within the muscle generate tension at any given moment. Therefore, this type of summation may generate submaximal contractions only.

An advantage of asynchronous motor unit summation is that the onset of muscle fatigue is significantly delayed because each motor unit has alternating periods of relaxation in which there is time for the restoration of energy supplies. The antigravity muscles of the back and legs employ asynchronous motor unit summation. These muscles are required to generate sustained submaximal contractions in order to maintain posture and body support over the course of the day.

Amount of tension developed by each contracting muscle fiber. As the amount of tension developed by each individual muscle fiber increases, the overall strength of skeletal muscle contraction increases. Three major factors determine the amount of tension developed by a contracting muscle fiber:

• Frequency of nerve stimulation

• Length of muscle fiber at the onset of contraction

• Diameter of muscle fiber

As mentioned previously, a single action potential lasting only 2 msec causes a muscle twitch that lasts approximately 100 msec. If the muscle fiber has adequate time to completely relax before it is stimulated by another action potential, the subsequent muscle twitch will be of the same magnitude as the first. However, if the muscle fiber is restimulated before it has completely relaxed, then the tension generated during the second muscle twitch is added to that of the first (see Figure 11.3). In fact, the frequency of nerve impulses to a muscle fiber may be so rapid that there is no time for relaxation in between stimuli. In this case, the muscle fiber attains a state of smooth, sustained maximal contraction referred to as tetanus.

The amount of tension developed by a muscle fiber during tetanic contraction can be as much as three to four times greater than that of a single muscle twitch. The mechanism involved with this increased strength of contraction involves the concentration of cytosolic calcium. Each time muscle fiber is stimulated by an action potential, Ca++ ions are released from the sarcoplasmic reticulum. However, as soon as the these ions are released, a

Figure 11.3 Muscle twitch summation and tetanus. A single action potential (represented by A) generates a muscle twitch. Because duration of the action potential is so short, subsequent action potentials may restimulate the muscle fiber before it has completely relaxed, leading to muscle twitch summation and greater tension development. When the frequency of stimulation becomes so rapid that no relaxation occurs between stimuli, tetanus occurs. Tetanus is a smooth, sustained, maximal contraction.

Figure 11.3 Muscle twitch summation and tetanus. A single action potential (represented by A) generates a muscle twitch. Because duration of the action potential is so short, subsequent action potentials may restimulate the muscle fiber before it has completely relaxed, leading to muscle twitch summation and greater tension development. When the frequency of stimulation becomes so rapid that no relaxation occurs between stimuli, tetanus occurs. Tetanus is a smooth, sustained, maximal contraction.

continuously active calcium pump begins returning Ca++ ions to the sarcoplasmic reticulum. Consequently, fewer Ca++ ions are available to bind with troponin and only a portion of binding sites on the actin become available to the myosin crossbridges. Each subsequent stimulation of muscle fiber results in release of more Ca++ ions from the sarcoplasmic reticulum. In other words, as the frequency of nerve stimulation increases, the rate of Ca++ ion release exceeds the rate of Ca++ ion removal. Therefore, the cytosolic concentration of calcium remains elevated. A greater number of Ca++ ions bind with troponin, resulting in a greater number of binding sites on the actin available to myosin crossbridges. As the number of cycling crossbridges increases, the amount of tension developed increases.

The amount of tension developed by a stimulated muscle fiber is highly dependent upon length of the muscle fiber at onset of contraction. This association between the resting length of the muscle fiber and tension development is referred to as the length-tension relationship. The sarcomere length at which maximal tension can be developed is termed the optimal length (Lo). In skeletal muscle, optimal length is between 2.0 and 2.2 mm. At this point, the actin filaments have overlapped all of the myosin crossbridges on the thick filaments (see Figure 11.4, point a). In other words, the potential for crossbridge cycling and tension development upon stimulation has been maximized.

Sarcomere length (|m)

Figure 11.4 The length-tension relationship. The length of the sarcomere prior to stimulation influences the amount of tension that may be developed in the muscle fiber. (a) The optimal length of the sarcomere is between 2.0 and 2.2 mm. At this length, actin overlaps all of the myosin crossbridges. The potential for crossbridge cycling and the tension that may be developed upon stimulation of the muscle fiber are maximized. (b) When the sarcomere is overstretched so that actin does not overlap the myosin crossbridges, then crossbridge cycling cannot take place and tension cannot be developed in the muscle fiber. (c) When the sarcomere is shortened prior to stimulation, thin filaments overlap each other and thick filaments abut the Z lines. Further shortening and tension development upon stimulation are markedly impaired.

Sarcomere length (|m)

Figure 11.4 The length-tension relationship. The length of the sarcomere prior to stimulation influences the amount of tension that may be developed in the muscle fiber. (a) The optimal length of the sarcomere is between 2.0 and 2.2 mm. At this length, actin overlaps all of the myosin crossbridges. The potential for crossbridge cycling and the tension that may be developed upon stimulation of the muscle fiber are maximized. (b) When the sarcomere is overstretched so that actin does not overlap the myosin crossbridges, then crossbridge cycling cannot take place and tension cannot be developed in the muscle fiber. (c) When the sarcomere is shortened prior to stimulation, thin filaments overlap each other and thick filaments abut the Z lines. Further shortening and tension development upon stimulation are markedly impaired.

If the muscle fiber is stretched prior to stimulation such that the actin filaments have been pulled out to the end of the thick filaments, no overlap is present between actin and the myosin crossbridges (see Figure 11.4, point b). In this case, no crossbridge cycling occurs and tension development is zero. Tension development is also impaired when the muscle fiber is allowed to shorten prior to stimulation (see Figure 11.4, point c). If the actin filaments overlap each other, fewer binding sites are available for the myosin cross-bridges. Also, if the thick filaments are forced up against the Z lines, further shortening cannot take place.

Interestingly, the range of resting sarcomere lengths is limited by the attachment of skeletal muscles to the bones. Because of this fixed orientation, skeletal muscles cannot overstretch or overshorten prior to stimulation. Typically, these muscles are within 70 to 130% of their optimal length. In other words, attachment to the bones ensures that the overlap of actin and myosin is such that crossbridge cycling approaches the maximum. As the number of cycling crossbridges increases, the strength of muscle contraction increases.

The diameter of the muscle fiber is influenced by two major factors:

• Resistance training

• Testosterone

Repeated bouts of anaerobic, high-intensity resistance training such as weight lifting cause muscle hypertrophy and increase the diameter of the muscle fiber. This form of training promotes synthesis of actin and myosin filaments and, as a result, the number of crossbridges available to cycle and develop tension is increased. Thus, larger muscles are capable of developing more powerful contractions.

Because muscle fibers in males are thicker than those found in females, these muscles are larger and stronger, even without the benefit of resistance training. This enlargement is due to effects of testosterone, a sex hormone found primarily in males. Testosterone promotes the synthesis of actin and myosin filaments in muscle fibers.

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.